1. Field of the Invention
Optical transmission systems operating at 10 Gb/s are now in the stage of practical implementation in trunk-line optical communications, but with rapid increases in recent years in the amount of information transmitted, due to ever increasing information communications as exemplified by the Internet, a further increase in communication capacity is demanded. One way to achieve this is to upgrade the transmission speed using time-division multiplexing techniques (including optical time-division multiplexing), and research and development of 40-Gb/s systems, the next generation of transmission systems succeeding the current 10-Gb/s systems, is actively underway both in Japan and abroad.
The present invention relates to a method of setting signal light at an optimum wavelength in an optical transmission system, and more particularly to a system of setting signal light at an optimum wavelength in an ultra high-speed time-division multiplexing optical transmission system so that chromatic dispersion in a transmission line, which varies from one repeater section to another and changes with time due to changes in ambient temperature and other environmental conditions, becomes minimum (zero) during system operation as well as at the start of system operation by using a tunable laser at the transmitting end.
2. Description of the Related Art
One of the factors limiting the transmission distance in a 40-Gb/s system is chromatic dispersion in a fiber-optic transmission line. Since chromatic dispersion tolerance is inversely proportional to the square of the bit rate, the chromatic dispersion tolerance, which is about 800 ps/nm at 10 Gb/s, is reduced by a factor of 16 to about 50 ps/nm at 40 Gb/s. According to the results of a transmission experiment conducted over a 50-km single-mode fiber (SMF) with zero dispersion at 1.3 xcexcm (chromatic dispersion=18.6 ps/nm/km, total dispersion=930 ps/nm) using a 40-Gb/s optical time-division multiplexing (OTDM) system (G. Ishikawa et al., ECOC ""96 ThC. 3.3), the dispersion compensation tolerance necessary to achieve a power penalty of 1 dB or less was 30 ps/nm. This means that in a 40-Gb/s system, the total dispersion in a transmission line must be very strictly controlled to within 30 ps/nm.
Furthermore, chromatic dispersion in a fiber-optic transmission line changes with time due to changes in ambient temperature, pressure, and other environmental conditions. For example, in the case of a dispersion-shifted fiber (DSF) with zero dispersion at 1.55 xcexcm, the amount of change of dispersion over a distance of 100 km, in the presence of temperature changes between xe2x88x9250xc2x0 C. and 100xc2x0 C., is estimated at 31.5 ps/nm from the following equation.
[Amount of change of dispersion]=[Temperature dependence of zero dispersion wavelength]xc3x97[Temperature change] xc3x97[Dispersion slope]xc3x97[Transmission distance]=0.03 (nm/xc2x0 C.)xc3x97150 (xc2x0 C.)xc3x970.07 (ps/nm2/km)xc3x97100 (km)=31.5 ps/nm
This value is approximately equal to the dispersion tolerance of 30 ps/nm, and must be considered properly in system design. The reason is that, even if the chromatic dispersion could be set to zero for xe2x88x9250xc2x0 C. at the start of system operation, if the temperature rose above 30xc2x0 C. during system operation, the criterion of 1 dB penalty could not be satisfied.
From the above discussion, one can see that the implementation of an ultra high-speed optical transmission system at 40 Gb/s or higher rates requires the construction of an xe2x80x9cautomatic signal wavelength optimization systemxe2x80x9d which
(i) sets the signal wavelength at the start of system operation so that the chromatic dispersion becomes minimum (zero), and
(ii) controls the signal wavelength during system operation so that the chromatic dispersion becomes minimum by adjusting to the variation with time in the dispersion value of the transmission line. Such an automatic signal wavelength optimization system is needed not only for a transmission system that uses 1.55-xcexcm dispersion-shifted fiber (DSF) having a low chromatic dispersion value at that wavelength, but also for a transmission system that uses 1.3-xcexcm zero dispersion single-mode fiber (SMF) in conjunction with a dispersion compensation technique.
As a method for measuring chromatic dispersion in an optical fiber, a pulse method or a phase method has traditionally been used that involves inputting a plurality of light beams of different wavelengths into the optical fiber and measuring group-delay differences or phase differences between the output beams. However, in order to constantly measure the dispersion using these methods during system operation, a pair of chromatic dispersion measuring devices must be provided for each repeater section. Further, to measure the dispersion amount without interrupting the transmission of data signal light, measuring light of a wavelength different from that of the data signal light must be wavelength-division multiplexed. Incorporating the pulse method or phase method into an optical transmission apparatus is not practical in terms of size and cost. Furthermore, when using a wavelength different from the wavelength of signal light, since the process involves estimating the dispersion value at the signal wavelength from the measured value at the measuring light wavelength, the result may lack accuracy. Therefore, a method that can measure the chromatic dispersion value directly from the signal light is desirable.
As a method that can achieve this, the present inventor proposed, in Japanese Patent Application No. 9-224056 entitled xe2x80x9cMethod and Apparatus for Controlling Chromatic Dispersion and Method of Detecting Amount of Dispersion,xe2x80x9d a method that utilizes the total-dispersion dependence of the intensity of a 40-GHz component contained in the baseband spectrum of an NRZ signal and an OTDM signal. More specifically, the method utilizes the characteristic that when the amount of total dispersion is zero, the intensity of the 40-GHz component is minimum and the eye opening at that time is maximum. The method proposed in Japanese Patent Application No. 9-224056 uses a variable dispersion compensator to reduce the amount of total dispersion to zero. This patent application also refers to a method that reduces the amount of total dispersion to zero by varying the signal wavelength using a tunable laser, but no mention is made of a specific control method to achieve it.
Accordingly, it is an object of the present invention to provide a method of setting signal light at an optimum wavelength in an optical transmission system.
According to the present invention, there is provided a method of setting signal wavelength in an optical transmission system, comprising the steps of: sweeping the signal wavelength over a first wavelength range before operation of the optical transmission system is started; determining an optimum value the wavelength based on the result of the sweeping over the first wavelength range; after operation of the optical transmission system is started, sweeping the wavelength over a second wavelength range which is centered about the optimum value and is narrower than the first wavelength range; and updating the optimum value for the wavelength based on the result of the sweeping over the second wavelength range.